1. Field of the Invention
The invention relates to a method and an apparatus for transmission of safe process signals between input and output units in a safety system.
2. Background of the Invention
Particularly in the case of systems which comprise standardized networks and have to satisfy stringent safety requirements, such as SIL 3 according to IEC Standard 61508 or, for example, Safety Category 4 in accordance with EN 954-1, the external circuitry of, for example, sensors and actuators must be designed on the basis of two channels, in order to ensure the necessary safety by means of the redundancy.
Such two-channel external circuitry is conventionally passed in the case of safety systems to similarly comparable, other redundant external circuitry, via separate signals from the sensors, which signals are normally transmitted via input/output units using a network or backplane bus of an integrated computer for control and/or logic processing. The control and/or logic processing then processes the two-channel signal circuitry by means of an appropriately matched emergency-off functional component, which must therefore furthermore be designed to have two channel inputs, and in consequence initiates a safe reaction. The safe reaction is often carried out directly at the controller, or is transmitted by means of a network and/or a backplane bus for the system-integrated computer to a corresponding output assembly, for example to an actuator.